[cygwin][vim] Modifications for .vimrc.

VIM_DISABLE_YCM=1
	To disable YCM and use vim-autocomplpop instead.

VIM_DISABLE_COVIM=1
	To disable loading CoVim plugin.

Other modifications:
1. Merge .vimrc from .vimrc.rich.
2. Hotkey modifies:
   <s-tab> to move to previous tab
   <leader><tab> to move to last focused tab
   <leader><f3> to save sesson
   <leader><c-]> using YCM to find define or declare
   <c-f12> to move to next hunk, or opening gblame
   <leader><f7> to start spelling check (compared to <f7> YCM diags)
3. Do not enable \ec by default.
4. not knowing why after entering VIM, the default mode is insert mode.
   therefore, always simulate keystroke <c-[> to return to normal mode.
Merge branch 'cygwin' of github.com:starleoda/env into cygwin